VIBRATIONALLY RESOLVED CARBON CORE EXCITATIONS IN ALKANE MOLECULES

Core-excitation spectra at the carbon K threshold of methane, ethane, and propane, as well as of the deuterated molecules, were studied by high-resolution photoabsorption using synchrotron radiation from the SX700/II monochromator at BESSY. The spectra contain signals from excitations to C 1s-1np Rydberg states and their vibrational substates, as well as from transitions to C 1s-1ns Rydberg states. The latter excitation is dipole forbidden in methane, but rendered possible by strong vibronic coupling. Transitions to vibrational substates could be identified via the observed isotope effects. The C 1s-1np Rydberg states were found to be split by a Jahn-Teller effect or by molecular-field interaction. C-H and C-C equilibrium distances as well as vibrational frequencies in the core-excited molecular states were derived within the Franck-Condon model from the vibrational fine structures of the C 1s(-1)3p Rydberg excitations.